From: richard.schneeman@...
Date: 2014-05-01T15:33:49+00:00
Subject: [ruby-core:62268] [ruby-trunk - Feature #9785] Feature Proposal: Dir.chdir Thread Safety

Issue #9785 has been updated by Richard Schneeman.


I think maybe the openat and family of *at calls is close to my original proposal but does not help for executing a script inside of a chdir block: https://github.com/heroku/hatchet/commit/f882d8920525df6c1dda5fbd5494ce03aaa7c592#diff-c8c936aa2a8d587bef4a4232e0028ed9L63. 

As my original proposal violates the basic assumptions of threads and CWD, I think this specific proposal can be closed. Maybe when support becomes better for those functions or if someone has a better idea of how to utilize them we can open up a new issue. Here is a related discussion from 2008 that I found: https://www.ruby-forum.com/topic/165079

----------------------------------------
Feature #9785: Feature Proposal: Dir.chdir Thread Safety
https://bugs.ruby-lang.org/issues/9785#change-46430

* Author: Richard Schneeman
* Status: Open
* Priority: Normal
* Assignee: 
* Category: core
* Target version: current: 2.2.0
----------------------------------------
I am proposing that `Dir.chdir` with a block be local to the current thread and any threads that are created inside of that block. `FileUtils.cd` and `FileUtils.chdir` should also behave the same way. 

Currently `Dir.chdir` will change the directory for the entire process. This makes writing a program that modifies different directories in threads very difficult. Here is some ruby code that demonstrates the problem:

```ruby
# /tmp/code.rb

require 'fileutils'

FileUtils.mkdir_p("/tmp/foo")
FileUtils.mkdir_p("/tmp/bar")


threads = []
threads << Thread.new do
  Dir.chdir("/tmp/foo") do
    puts "Thread in Dir.chdir('/tmp/foo') pwd: #{`pwd`}"
  end
end


threads << Thread.new do
  puts "Thread without Dir.chdir        pwd: #{`pwd`}"
end

threads.map(&:join)
```

When you run it you get different results:

```
$ ruby /tmp/code.rb
Thread without Dir.chdir        pwd: /tmp
Thread in Dir.chdir('/tmp/foo') pwd: /private/tmp/foo

$ ruby /tmp/code.rb
Thread in Dir.chdir('/tmp/foo') pwd: /private/tmp/foo
Thread without Dir.chdir        pwd: /private/tmp/foo
```

This is because Dir.chdir is not limited to the scope of the block but rather changes the working directory globally for the entire process including different threads. 

Threads in MRI are very good for reading and writing to the disk, however many times a programmer wishes to read or write to disk they will want to use `Dir.chdir`. The current behavior of `Dir.chdir` prevents a programmer from changing directory inside of threads and can be very confusing for anyone who does not know this behavior. 

For a better programming experience either we can make `Dir.chdir` thread aware, or introduce a new way to change the directory inside of a new thread such as `Dir.threadsafe_chdir`, I believe the first option is the best.




-- 
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